A. Field of the Invention
The invention generally concerns the use of aluminosilicate zeolite catalysts for the conversion of chloromethane to olefins. In particular, an aluminosilicate zeolite catalyst SSZ-13 having a chabazite zeolite structure can be used to convert an alkyl halide feed that is substantially free of oxygenates to light olefins.
B. Description of Related Art
Light olefins such as ethylene and propylene are used by the petrochemical industry to produce a variety of key chemicals that are then used to make numerous downstream products. By way of example, both of these olefins are used to make a multitude of plastic products that are incorporated into many articles and goods of manufacture. FIGS. 1A and 1B provide examples of products generated from ethylene (FIG. 1A) and propylene (FIG. 1B). Currently, the main process used to prepare light olefins is via steam cracking of naphtha. This process, however, requires large amounts of naphtha, which in-turn, is obtained from the distillation of crude oil. While this process is viable, its reliance on crude oil can be a rate-limiting step and can increase the manufacturing costs associated with ethylene and propylene production. Thus, other feed sources such as oxygenates and alkyl halides have been investigated as possible feedstocks.
Various publications have been directed to the use of zeolitic or aluminosilicate catalysts in oxygenate to olefin (OTO) processes such as methanol to olefin (MTO). By way of example, U.S. Pat. No. 7,148,172 to Strohmaier et al. describes a MTO process that can convert a feed that includes oxygenates (e.g., methanol or ethanol) and alkyl halides having 3 to 10 carbon atoms to olefins rich in ethylene and propylene by contacting the feed with a low acidity chabazite material (i.e., a silica and alumina with a molar ratio of silica to alumina in excess of 100, such as 265). However, due to the remaining acidic catalytic sites, dehydration or partial dehydration of methanol or C3-C10 alkyl halides is possible, and by-products such as di-olefins, dimethyl ether and formaldehyde can be formed. Another problem associated with MTO processes is that when the oxygenate-based feedstock contains ethanol, acetaldehyde (e.g., which is formed by selective dehydrogenation of ethanol) or other oxidized compounds can be formed as by-products. Under certain conditions, formaldehyde can further react to form methyl formate and, in some cases, hydrogen and carbon monoxide. To inhibit the formation of by-products when oxygenates are present, the MTO catalysts can be treated with dopants (e.g., metal oxides) to minimize the formation of acetaldehyde. By way of example, U.S. Pat. No. 7,829,751 to Levin et al. describes a methanol/ethanol process for producing olefins from an oxygenate feed that includes at least 5 wt. % ethanol in which the feed is contacted with a catalyst composition that includes an aluminosilicate catalyst and a basic metal oxide co-catalyst. The basic metal oxide co-catalyst is used to minimize the formation of oxygenate by-products (e.g., acetaldehyde).
Other methods to generate olefins that use alkyl halides as a feed source use silicoaluminophosphate zeolite catalysts (e.g., SAPO-34) or catalysts having a pentasil structure (e.g., ZSM-5). By way of example, Wei et al. in New route for light olefins production from chloromethane over HSAPO-34 molecular sieve, Catalysis Today, Vol. 106, October 2005, pp. 84-89 describes the use of a calcined HSAPO-34 zeolites to convert chloromethane to light olefins. However, these catalysts suffer from less than optimal selectivity to a desired olefin (e.g., propylene) and rapid catalyst deactivation. In Xu et al. in Fluoride-treated HZSM-5 as a highly selective stable catalyst for the production of propylene from methyl halides, Journal of Catalysis, Vol. 295, November 2012, pp. 232-241, calcined HZSM-5 catalysts were used and deactivated quickly. The catalysts required doping with fluoride to improve catalytic performance.
One of the problems seen with the currently available catalysts used to produce olefins is that such catalysts have relatively low catalyst activity. This results in an inefficient process that increases the time and expenses associated with producing olefin products. Still further many of the currently used processes rely on the presence of oxygenate compounds in the reaction feed, thereby further increasing production costs, and also introducing the possibility of the formation of unwanted by-products from said oxygenates.